Accurate duplication of the centrosome precisely once per cell cycle is required for formation of the mitotic spindle, the apparatus responsible for proper segregation of chromosomes during mitosis. Defects in chromosome stability contribute genetic instability in cancer cells and centrosome defects are evident in many cancers. The centrosome serves as a microtubule organizing center (MTOC) and spindle pole. A detailed understanding of its duplication mechanism and cell cycle regulation will require a robust in vitro duplication system. We propose to investigate whether basal body duplication in the ciliated protozoan Tetrahymena can be the basis of an in vitro system. The basal body is an MTOC that forms cilia or flagella. Basal bodies are very similar to centrioies that are found in centrosomes. The duplication events of the centrioles and basal bodies are morphologically similar, and in fact some organisms convert the organelle from centriole to basal body and back during the course of the cell cycle. Furthermore, it is known that the two organelles share a number of components, including proteins involved in duplication of the organelles. Therefore, analysis of basal body duplication will provide important insights into centriole duplication, the event at the heart of centrosome duplication. Furthermore, we believe that Tetrahymena uniquely offers the opportunity to develop an in vitro basal body duplication system. Surprisingly, the basal body of Tetrahymena has received little attention in recent years even as tools for analysis of the organism have become progressively more sophisticated. We have chosen the R21 funding mechanism as a way to support our initial efforts to demonstrate that (1) basal body duplication in Tetrahymena can be studied using available tools and methods, and (2) to attempt to establish an in vitro basal body assembly assay.